Standard Cell Characterization

Introduction: -

In the present scenario, it is very hard to design a circuit using full custom methodology because of the increase in the complexity due to decrease in the feature size. So, most of companies are following the semi-custom approach to reduce the cost & time to market which are the most important parameters for any successful products.

To follow the approach of semi-custom, designer need a characterized library of standard cells e.g. logic gates, data buses, sequential building blocks like F/F's etc. That's why, many companies & fabrication plants provide std. cell library with the data sheets containing information about timing, area & power parameters.

Overview: -

In this report, I will discuss the requirement of std. cells & basic flow for their characterization. The topics of discussion are given below:

 Basics

* Reasons for Characterization

* Characterization Flow

* Characterization Parameters

* Models

* Measurement & Verification

* Library Formats

* Summary

* Sources

Basics: -

The use of the standard cells can be summarized as given below:

 Logic units of similar geometry (same height)

 Implement basic logic (NAND, NOR, INV, FF, LATCH, complex gates)

 Usually come in libraries for a specific technology

From the above points, we can conclude that std. cells provide repeatability during layout design which decreases the complexity & make layout formation easy. Also, std. cells are technology dependent so just by selecting library of one technology; we can build any design depending on available cells as shown below.

Reasons for Characterization: -

The reasons for the characterization are given below: -

 Extraction of functionality is complicated

 Functional/Delay simulation takes way too long

 Power extraction for a whole chip takes too long

 Automatic detection of timing constraints (e.g. Setup time) is

difficult.

Solution: To solve this problem, a simple model for delay, function,

constraints and power on cell/gate level is used which provides

different characteristics of the cell & thus called cell characterization.

Characterization Flow: -

The characterization flow for std. cell is a major issue. Many aspects of the characterization demand special attention. The major steps are given below: -

 Net list Extraction: In this, the layout for the cell to be characterized is made using any good layout editor for a specific technology & then verified for the technology rules violations & interconnections. After verifying the design, the parasitic extraction is done in which resistance; capacitance & other physical parasitic are being extracted from the design.

 Specification of parameters: After the extraction, some default, process & design dependent parameters are defined. For example, doping density, voltage, temperature. Fan out etc.

 Model selection and specification: - After specifying the parameter values, different models are selected depending on their accuracy, complexity & requirement. A Model estimates the timing, area, power & noise parameters of the std. cell. In real sense, Models are mathematical equations used to calculate the o/p parameters.

 Measurement: - The o/p provided by the simulator is then measured to extract the required characterization parameters. The o/p may be in different forms e.g. graphical view, bar chart and shmoo plot etc. From these characteristics, different parameters like area, power and timing are calculated for the std. cell.

 Model Generation: - Using these parameters, a characterized o/p model for the std. cell is prepared in a format to which a design tool can understand. All the measured parameters are attached to std. cell.

 Verification: - After the characterization of the std. cell, it is verified for different parameters by building a design from it. If it gives the desired result then the library is sign out. But if there is any error then flow is repeated from different stages depending on the severity of the error.

Characterization Parameters: -

During the characterization of the std. cell, different parameter values are decided which are used in different mathematical models for timing, power & area. A summary of these parameters are given below: -

Global Parameters

 PVT Corner Selection (Process, Voltage, Temperature)

 Standard Load Definition

 Default Definition (Default value to Parameters)

 Threshold values (Transition Thresholds)

 Limits (max. Output Load, max. Transition time)

 Wireload Models (area, resistance, capacitance, slope and fan-out)

Simulation Parameters

 Functionality

 Delay simulation

Area, Power, Timing constraints

For Power